Along with the development of electronic systems, the sizes of different electronic elements have been constantly reduced, and the number of transistors per unit area in a chip has been increased accordingly. Modularizing all circuits in a product and integrating the functions of these modules into a single chip is one of the trends in the future development of electronic systems. Clock signal perform synchronization in various electronic systems. In a portable electronic device or an electronic biomedical product, a clock source with low power consumption and small surface area may extend the lifespan of the product. Presently, a clock signal is usually generated using a crystal oscillator, micro electro mechanical systems (MEMS), or through a CMOS process. The frequency of a crystal oscillator or MEMS is stable at different temperatures and voltages. As to the circuit system structure, an oscillating circuit realized through a CMOS process possesses integration superiority over oscillating circuits realized through other techniques.
Existing CMOS oscillators come in three types of main structures. The first type of main structure adopts a LC oscillator for high-frequency system operations. The second type of main structure adopts a relaxation oscillator, in which the frequency oscillation is controlled by charging/discharging loads including at least a resistor and/or at least a capacitor. The third type of main structure is an all-digital oscillating structure, in which a digital signal processor and a clock generator are integrated into a single chip so that the system area is effectively reduced. Because the frequency stability of a CMOS circuit is sensitive to temperature, how to respond to an environmental variation in a CMOS structure is a subject to be resolved.